The rotating steel shafts of large turbine-generators must be continuously grounded to prevent damage to shaft bearings from electrostatic and dissymmetry voltages. Operation of this rotating machinery can result in electrical charge buildup on the rotor or shaft, which is supported at spaced-apart locations by the bearings. Electrostatic charges can be transferred to the rotating system by small water particles within the low-pressure turbine; dissymmetry voltages may be generated within the generator shaft by magnetic irregularities in the stator core. The shaft itself actually rides on a thin film of oil or other suitable lubricant in the bearing, and accordingly is electrically insulated from ground potential. The buildup of an excessive electrical charge on the shaft can cause a discharge through the oil film, resulting in damage to the bearings.
In order to prevent such discharge, and in order to ground the rotating shaft, shaft grounding devices (SGD) are placed in continuous contact with the rotating shaft as it rotates, to afford a discharge path to ground through the bearing pedestals for the electrical charge buildup. Typically, these devices are placed between the last low-pressure turbine and the generator. The SGD functions by draining electrostatic charge to the frame, and confine dissymmetry voltages to that part of the rotating shaft within the generator and exciter. The generator and exciter bearings are protected from the destructive effects of dissymmetry voltages by being insulated from the system frame. During the operating life of the grounding device, an insulating film buildup may occur, thereby interrupting the continuous path to ground for, the electrical charge. A voltage builds up on the shaft to a certain threshold value whereby discharge through the oil film takes place, which can lead to bearing damage. When the SGD fails to make effective and continuous contact with the shaft, problems such as bearing and journal pitting can reduce bearing life.
Maintaining effective, long-term contact is difficult because of the high surface velocity of the steel shaft, and contamination to the shaft by oil, water droplets and dirt. Because of this likelihood of damage to the bearings, frequent periodic inspections of the SGDs have been required in the past.
To reduce the manpower required for these periodic maintenance inspections, on-line monitoring systems have been used to monitor shaft voltage, or to detect electrical discharge which may occur and which is a symptom of poor shaft grounding. Recently, an active shaft grounding system has been adopted, which reduces SGD service required by actively compensating for voltage developed across the shaft SGD interface. The active shaft grounding system compensates for poor contact at the SGD-to-shaft interface, but on-line service may still be required when this voltage exceeds the limit of the active shaft grounding system active drive circuit. Also, contamination of the oil film can also render the active shaft grounding system unable to effectively do its intended purpose. The active shaft grounding system is disclosed in U.S. Pat. No. 4,873,512 issued on Oct. 10, 1989, to Miller, and assigned to the present assignee. The Miller patent is hereby incorporated by reference.
The continuing need for on-line service between periodic maintenance inspections is of concern, because of the high speed of the rotating shaft and the typically hot, oily environment of the confined work space where the SGDs are located within the turbine-generator system. Thus, it is desirable that entry by plant personnel into this area be kept to a minimum.
It is therefore an object of the present invention to provide an automatic means to re-establish shaft grounding in the case of shaft grounding device fouling.
It is a further object of the present invention to prevent bearing damage by electrical discharge from the shaft through the oil film to the bearings.
It is a further object of the present invention to minimize entry of plant personnel into the hostile environment of an operating turbine-generator system.